Method for measuring ethanolamine phosphate

ABSTRACT

To provide a method for simply measuring ethanolamine phosphate in a sample, and a reagent, kit, program and the like useful in the method. 
     A measurement method of ethanolamine phosphate includes a first step of adding an enzyme, which can catalyze a reaction that forms acetaldehyde from ethanolamine phosphate, to a sample, and conducting a first enzymatic reaction to form acetaldehyde, phosphoric acid and ammonia; and a second step of quantifying at least one of the resultant acetaldehyde, phosphoric acid and ammonia to determine an amount of the ethanolamine phosphate in the measurement sample.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Division of application Ser. No. 14/237,488, filed Feb. 6, 2014, which is a National Stage Application of PCT/JP2012/078749, filed Nov. 6, 2012, which claims foreign priority from Japanese Patent Application Number 2011-246881, filed Nov. 10, 2011.

TECHNICAL FIELD

This relates to a measurement method of the ethanolamine phosphate level in a sample.

BACKGROUND ART

Depression is a kind of mood disorder, and is accompanied by “depressed mood” and “loss of interest or pleasure” as primary symptoms. According to a survey among medical institutions in Japan, more than 700,000 depressive patients were considered to exist in 2008. However, a diagnosis of depression depends in a large part on the subjective view of a physician or psychologist about emotional aspects of a patient or on the subjective view and response of the patient himself or herself, and is hardly considered to be made through an objective determination. Attempts have, therefore, been made in recent years to identify a component in a body fluid of a patient as an objective guideline for the diagnosis of depression.

It has been reported to be able to determine, as a predictive marker for depression, the level of triptophan or its degradation product, the expression level of a specific gene, or the like in a body fluid (Patent Documents 1 and 2). In the meantime, the present inventors found that the ethanolamine phosphate level in blood is useful as a biomarker for diagnosing depression (Patent Document 3).

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: WO-A-2006/105907

Patent Document 2: JP-A-2008-253258

Patent Document 3: WO-A-2011/019072

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

An ethanolamine phosphate level is highly reliable and excellent as a biomarker for depression, but its measurement has been limited to a measurement method that uses a CE-TOFMS (capillary electrophoresis time-of-flight mass spectrometry) system. However, measurements by CE-TOFMS are time consuming (30 to 40 minutes). Moreover, CE-TOFMS systems are expensive and precision analysis equipment. Because of this, CE-TOFMS systems are equipped in only a limited number of universities and analysis institutes. There is, accordingly, an outstanding need for establishing a quick and simple measurement method, which allows even local hospitals and clinics to measure an ethanolamine phosphate level as a biomarker for depression.

Objects of the present invention are, therefore, to provide a method for simply measuring the ethanolamine phosphate level in a sample, and also, a reagent, kit, program and the like useful in the method.

Means for Solving the Problem

The above-described objects can be achieved by the present invention to be described hereinafter. Described specifically, the present invention provides a measurement method of ethanolamine phosphate, comprising a first step of adding an enzyme, which can catalyze a reaction that forms acetaldehyde from ethanolamine phosphate, to a sample, and conducting a first enzymatic reaction to form acetaldehyde, phosphoric acid and ammonia; and a second step of quantifying at least one of the resultant acetaldehyde, phosphoric acid and ammonia to determine an amount of the ethanolamine phosphate in the measurement sample.

The enzyme may preferably be an enzyme having a GabT domain.

Advantageous Effects of the Invention

According to the present invention, there are provided a method for simply measuring the ethanolamine phosphate level in a sample and a reagent useful in the method, and a diagnostic agent capable of simply diagnosing depression.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing ethanolamine phosphate (EAP)-degrading activity of a Pan lysate.

FIG. 2 illustrates a graph showing analysis results of another Pan lysate by anion chromatography (lower) and a graph showing SDS-PAGE results of respective factions (upper).

FIG. 3 illustrates a graph showing analysis results of some fractions of the another Pan lysate by hydrophobic chromatography (lower) and a graph showing SDS-PAGE results of respective sub-factions (upper).

FIG. 4 illustrates a graph showing analysis results of some other fractions of the another Pan lysate through a hydroxyapatite column (lower) and a graph showing SDS-PAGE results of respective sub-factions (upper).

FIG. 5 is a graph showing measurement results of ethanolamine phosphate-degrading enzyme activity in GabT expression systems.

FIG. 6 is a diagram showing alignment results of GabTs of SEQ ID NOS: 1, 2, 4, 5, 6, and 7.

MODES FOR CARRYING OUT THE INVENTION

In the method of the present invention for the measurement of an ethanolamine phosphate level, an enzyme which may hereinafter be called “the first enzyme” is used in the first step. The first enzyme catalyzes a reaction, which hydrolyzes ethanolamine phosphate (CAS Registry Number: 1071-23-4) to form acetaldehyde, phosphoric acid and ammonia and which is represented by the below-described reaction equation (1). This reaction may hereinafter be called “the first enzymatic reaction”.

Heretofore, no enzyme has been identified yet to catalyze the one-step reaction of equation (1). However, the present inventors isolated an enzyme having catalyst activity for the above-described reaction from a Pantoea ananatis strain, and identified that enzyme to be γ-aminobutyrate aminotransferase (GabT).

In the measurement method of the present invention, at least one of acetaldehyde, phosphoric acid and ammonia formed through the enzymatic reaction is quantified in the second step to determine the amount of ethanolamine phosphate in the sample. Only one of acetaldehyde, phosphoric acid and ammonia may be subjected to the quantification, but the quantification of two or more of them makes it possible to reduce a measurement error.

When quantifying acetaldehyde in the second step, it can be quantified with good sensitivity by making use of the property of reduced nicotinamide adenine dinucleotide (NADH) that it absorbs 340 nm wavelength ultraviolet rays well. Specifically, a method comprising the following two sub-steps can be mentioned.

As the first sub-step, acetaldehyde dehydrogenase (ALDH) and oxidized nicotinamide dehydrogenase (NAD⁺) are added to the measurement sample after the above-described first step to oxidize acetaldehyde in the sample to acetic acid [see equation (2)].

In this reaction, NAD⁺ is reduced to NADH. Because only NADH strongly absorbs 340 nm ultraviolet rays, the measurement of the absorbance at 340 nm of the measurement sample after the reaction as the next sub-step can lead to quantifying the amount of acetaldehyde formed in the measurement sample after the first step, and from the value so quantified, the amount of ethanolamine phosphate in the measurement sample can be determined. Upon measurement of the acetaldehyde level in the second step, a commercial acetaldehyde quantification kit (for example, “F-Kit Acetaldehyde”, product of Roche Diagnostics GmbH) can be used.

In the first step, it is preferred to use a coenzyme in combination so that the efficiency of the enzymatic reaction can be increased further. As a preferred coenzyme, pyridoxal phosphate can be mentioned.

No particular limitation is imposed on the enzyme for use in the first step insofar as it can catalyze the reaction of equation (1) that hydrolyzes ethanolamine phosphate to form acetaldehyde, phosphoric acid and ammonia in one step. As examples of the enzyme having this enzyme activity, enzymes having a GabT domain can be mentioned. The term “GabT domain” means an amino acid sequence region defined by GXXXBADEBQXGFZRXG, wherein X means an amino acid, B denotes a branched amino acid selected from the group consisting of I, L and V, and Z signifies G or A.

As will be made apparent in examples to be described subsequently herein, the present inventors found that in a wide variety of organism species including archaebacteria, prokaryotes and eukaryotes such as plants and animals, their amino acid sequences have high homology of at least 90% with the GabT domain of γ-aminobutyrate aminotransferase. As examples of the enzyme which has enzyme activity capable of catalyzing the reaction of equation (1) and can be suitably used in the first step, proteins with amino acid sequences having at least 90% of homology with the GabT domain can be mentioned.

Specific examples of the enzyme, which has enzyme activity capable of catalyzing the reaction of equation (1) and has a GabT domain, include a protein (SEQ ID NO: 1 in the accompanying sequence listing) having a GabT domain derived from Pantoea ananatis and a protein (SEQ ID NO: 2) having a GabT domain derived from E. coli, both of which were identified by the present inventors. For use in the first step of the measurement method according to the present invention, the enzyme can be any protein insofar as it has activity to catalyze the reaction of equation (1). Therefore, the enzyme may also be a protein consisting of an amino acid sequence similar to the amino acid sequence represented by SEQ ID NO: 1 or 2 except that one or a few amino acids have been deleted, substituted or added, or a protein consisting of an amino acid sequence having at least 90% of homology with the amino acid sequence represented by SEQ ID NO: 1 or 2. The enzyme may be in an unpurified form, and even a partial purification product or a cell lysate containing the enzyme is usable insofar as it has the enzyme activity. As an example of the cell lysate, a supernatant obtained by the centrifugation of a Pantoea ananatis lysate can be mentioned.

The method of the present invention for the measurement of an ethanolamine phosphate level is very simple in procedure, and can be incorporated as a system in existing clinical chemistry equipment. Described specifically, a measurement sample is set on a measurement system. After the first enzyme is added to the measurement sample and acetaldehyde, phosphoric acid and ammonia are formed, at least one of the acetaldehyde, phosphoric acid and ammonia is quantified. As this method involves no reliance on human subjective view, it is suited for routine processing by the measurement system and can lead to quickly and simply measuring a large number of samples. Such processing can also be performed by making a computer, which is incorporated in the measurement system, read a program that allows the measurement system to conduct the first step of adding the enzyme to each measurement sample and the second step of quantifying at least one of acetaldehyde, phosphoric acid and ammonia.

As described above, there is a significant correlation between the concentration of ethanolamine phosphate in plasma and depression (see Patent Document 3). By further adding, to the above-described program, a step of determining, based on the measurement value of ethanolamine phosphate level in a measurement sample, whether or not the subject who provided the measurement sample is a depressive patient and another step of outputting the measurement result so obtained, it is hence possible to diagnose whether or not the subject who provided the measurement sample is a depressive patient.

The program maybe recorded on a recording medium readable by a computer, or on a recording medium in a computer attached to the measurement system. The recording medium may be, but is not limited specifically to, a hard disk, CD, DVD, USB memory, Floppy® disk, or the like.

According to the present invention, there is also provided an ethanolamine phosphate-measuring reagent which contains an enzyme capable of catalyzing a reaction that forms acetaldehyde from ethanolamine phosphate. As the enzyme capable of catalyzing the reaction that forms acetaldehyde from ethanolamine phosphate, it is possible to use an enzyme similar to that employed in the first step of the measurement method of the ethanolamine phosphate level. Described specifically, proteins consisting of the amino acid sequences described under SEQ ID NO: 1 and SEQ ID NO: 2 in the sequence listing can be mentioned.

Preferably, the ethanolamine phosphate-measuring reagent may further contain a coenzyme for the enzyme to further improve the react ion efficiency of the enzyme. As a specific example of the coenzyme, pyridoxal phosphate can be mentioned. No particular limitation is imposed on the enzyme insofar as it can catalyze the reaction of the equation (1) that hydrolyzes ethanolamine phosphate to form acetaldehyde, phosphoric acid and ammonia in one step. As an example of the enzyme that has the enzyme activity, an enzyme having a GabT domain can be mentioned. As an alternative, the enzyme may be a protein having an amino acid sequence region having at least 90% of homology with the above-described GabT domain.

Since there is a significant correlation between the concentration of ethanolamine phosphate in plasma and depression as described above, the ethanolamine phosphate-measuring reagent can be used as a depression diagnostic agent.

According to the present invention, there is also provided an ethanolamine phosphate-measuring kit comprising a container with an enzyme, which can catalyze a reaction that forms acetaldehyde from ethanolamine phosphate, separately contained therein. Because the suffering from depression can be determined from the concentration of ethanolamine phosphate in plasma as described above, the use of the ethanolamine phosphate-measuring kit according to the present invention makes it possible to conduct screening of depressive patients from those subjected to an examination or check-up under a physical examination program or employees' in-house medical check-up program, to allow a non-specialist physician to conduct a diagnosis of a patient with suspected depression, or to allow a specialist to conduct a diagnosis along with an interview, to assess the effectiveness of treatment and to determine a strategy of treatment.

Preferably, the ethanolamine phosphate-measuring kit may further comprise another container with a coenzyme for the enzyme, said coenzyme being contained separately in said another container. As an example of the coenzyme, pyridoxal phosphate can be mentioned. No particular limitation is imposed on the above-described enzyme insofar as it can catalyze the reaction of equation (1) that forms acetaldehyde, phosphoric acid and ammonia in one step. As an example of the enzyme having the enzyme activity, an enzyme having a GabT domain can be mentioned. As an alternative, the enzyme maybe a protein having an amino acid sequence region having at least 90% of homology with the above-described GabT domain.

EXAMPLES

1. Preparation of Cell Lysate with Ethanolamine Phosphate-degrading Enzyme Contained Therein

As cell strains having ethanolamine phosphate lyase activity, three Erwinia strains (Erwinia carotovora, Pantoea ananatis LMG 20103 strain, and Erwinia milletiae) were selected, and were purchased from Incorporated Administrative Agency, National Institute of Agrobiological Sciences. The three strains were subjected to liquid culture in portions of an oligotrophic synthetic medium with ethanolamine contained as a single nitrogen source therein. Resulting colonies were spread on LB plates and cultured overnight at 30° C., and colonies were obtained on the respective plates.

From each plate, two to three colonies were harvested, suspended in a liquid medium (2 to 5 mL), and cultured at 30° C. and 170 rpm. As pronounced proliferation was observed only on Pantoea ananatis out of the above-described three strains, that cell strain was cultured until OD₆₀₀=1.0, and subsequent to the culture, centrifugation was conducted to harvest its cells.

The thus-obtained cell strain was suspended in Buffer A [a solution of 2-mercaptoethanol (final concentration: 1 mM) in tris-hydrochloric acid buffer (final concentration: 50 mM), pH 7.5], cell disruption was conducted by sonication, and subsequent to centrifugation, a supernatant was collected to prepare a lysate (hereinafter called “the Pan lysate”).

2. Measurement of Ethanolamine Phosphate-Degrading Enzyme Activity of Pan Lysate Referential Example 1

To a reaction solution (1 mL) containing ethanolamine phosphate (final concentration: 2 mM) as a reaction substrate and pyridoxal phosphate (final concentration: 2 mM) as a coenzyme, the Pan lysate was added (final volume ratio: 5%), and the resulting mixture was incubated at 30° C. One hour later, the mixture (200 μL) was deproteinized by filtration (“Microcon®”, Mw: 5k) to terminate the reaction. Subsequently, the concentration of ethanolamine phosphate in the filtrate was measured using a quadropole mass spectrometer (“Quadropole LC/MS 6140” manufactured by Agilent Technologies, Inc.). A similar measurement was conducted on the sample after 4 hours and 20 hours from the initiation of the experiment. Results are indicated by black dots () in FIG. 1.

Referential Example 2

Measurements were conducted as in Referential Example 1 except for the use of a 10-fold dilute solution of the Pan lysate in Buffer A in place of the Pan lysate. Results are indicated by black triangles (▴) in FIG. 1.

Referential Example 3

Measurements were conducted as in Referential Example 1 except for the use of a 100-fold dilute solution of the Pan lysate in Buffer A in place of the Pan lysate. Results are indicated by black rhombi (♦) in FIG. 1.

Referential Example 4

Measurements were conducted as in Referential Example 1 except for the use of Buffer A, as a negative control, in place of the Pan lysate. Results are indicated by black squares (▪) in FIG. 1.

From the results shown in FIG. 1, it is evident that the ethanolamine phosphate in each measurement sample was degraded and decreased in concentration with time by the enzyme in the Pan lysate. It was, therefore, demonstrated that an ethanolamine phosphate-degrading enzyme was contained in the Pan lysate.

3. Isolation of Ethanolamine Phosphate Lyase Derived from Pantoea ananatis

With the same culture medium and under the same conditions as in Item 1 described above, Pantoea ananatis was subjected to mass culture on the liquid medium. The thus-obtained cells were suspended in Buffer B [a solution of magnesium chloride (final concentration: 10 mM) in tris-hydrochloric acid buffer (final concentration: 20 mM), pH 7.5], cell disruption was conducted by sonication, and then, centrifugation was performed to obtain a supernatant fraction. The fraction was first subjected to elution fractionation under a gradient of sodium chloride (100-500 mM) by anion chromatography (“DE-52”, product of GE Healthcare Japan Corporation), and then, SDS-PAGE was conducted on respective fractions. The respective fractions were added to portions of a tris-hydrochloric acid buffer (final concentration: 20 mM; pH 7.5), which contained ethanolamine phosphate (final concentration: 1 mM) and pyridoxal phosphate (final concentration: 1 mM), such that the final volume ratios of the fractions reached 30%. After the resulting mixtures were separately subjected to the reaction at 30° C., acetaldehyde dehydrogenase (ALDH) and oxidized nicotinamide dehydrogenase (NAD⁺) were added to the respective samples after the reaction according to the manual of a commercial acetaldehyde quantification kit (“F-kit Acetaldehyde”, product of Roche Diagnostics K.K.). Subsequent to the reaction of equation (2), the absorbances at 340 nm were measured. From each absorbance, the acetaldehyde in the corresponding sample after the reaction of equation (1) was quantified to determine the ethanolamine phosphate-degrading enzyme activity in the sample before the reaction of equation (1). The results of SDS-PAGE and the measurement results of ethanolamine phosphate-degrading enzyme activity are shown in an upper part and lower part of FIG. 2, respectively, such that each fraction in the upper graph and its corresponding fraction in the lower graph are aligned in the same column. The numbers in the lowest row are collected fraction numbers. “ft” means a flow-through, “w” denotes a wash solution (Buffer C-1) containing sodium chloride at 100 mM in Buffer B, and “wo” signifies awash solution (Buffer C-2) containing sodium chloride at 500 mM in Buffer. As shown in FIG. 2, strong ethanolamine phosphate-degrading enzyme activity was confirmed on the collected fractions numbered from 12 to 18. Further, it was considered from the results of SDS-PAGE that the bands around 75 kDa, 45 kDa and 40 kDa were likely those of ethanolamine phosphate-degrading enzymes.

The fractions numbered from 12 to 18 were then combined together, and the resulting solution was subjected to elution fractionation under ammonium sulfate (1-0 M) by hydrophobic chromatography (“Ether-650”, product of Tosoh Corporation). As eluents, Buffer B and a solution (Buffer D-1) containing ammonium sulfate at 1 M in Buffer B were used. Similar to the above-described anion chromatography, SDS-PAGE and absorbance measurements using the acetaldehyde quantification kit were conducted on the respective fractions. Results are shown in FIG. 3. As shown in FIG. 3, high ethanolamine phosphate-degrading enzyme activity was confirmed on the collected fractions numbered from 7 to 10. Further, it was considered from the results of SDS-PAGE that the bands around 75 kDa, 45 kDa and 25 kDa were likely those of ethanolamine phosphate-degrading enzymes.

The fractions numbered from 7 to 10 in the above-described hydrophobic chromatography were collected, and were subjected to fractionation through a hydroxyapatite column (“HT-Gel”, product of Bio-Rad Laboratories, Inc.). As eluents, a 10 mM potassium phosphate solution (pH 9.0) (Buffer E-1) and a 300 mM potassium phosphate solution (pH 9.0) (Buffer E-2) were used. Similar to the above-described anion chromatography, SDS-PAGE and absorbance measurements using the acetaldehyde quantification kit were conducted on the respective fractions. Results are shown in FIG. 4. As apparent from FIG. 4, high ethanolamine phosphate-degrading enzyme activity was confirmed on the collected fractions of numbers w and 1. Further, it was considered from the results of SDS-PAGE that the bands around 45 kDa and 25 kDa were likely those of ethanolamine phosphate-degrading enzymes.

From the results of the fractionation by the three kinds of chromatography, the three bands of 75 kDa, 45 kDa and 25 kDa were selected as candidates for ethanolamine phosphate-degrading enzymes.

In each of the experiments described above, acetaldehyde was formed by the addition of the cell lysate of Pantoea ananatis to the solution that contained ethanolamine phosphate. It has, therefore, been confirmed that the ethanolamine phosphate-degrading enzymes described above are enzymes capable of catalyzing the reaction of equation (1).

4. Identification of Ethanolamine Phosphate Lyase Derived from Pantoea ananatis

The three bands selected in Item 3 described above were collected from the SDS-PAGE gel, and the determination of their amino acid sequences was contracted to a custom service provider (Hokkaido System Science Co., Ltd.). As a result, those bands were all recognized to have high homology with the below-described proteins, respectively, and from their molecular weights and N-terminal amino acid sequences, were also confirmed to correspond to the following proteins, respectively.

-   -   75 kDa: phenylalanyl-tRNA synthesis enzyme, β-subunit (Pantoea         vagans C9-1)     -   45 kDa: GabT (Pantoea ananatis LMG20103) shown as SEQ ID NO:1     -   25 kDa: WrbA (Pantoea ananatis LMG20103)

Among the above-described proteins, the phenylalanyl-tRNA synthesis enzyme and WrbA have been reported to catalyze reactions different from deamination and dephosphorylation. They were, accordingly, excluded from the candidates for ethanolamine phosphate-degrading enzymes.

The remaining gene for GabT has been confirmed to exist in the genomes of a relatively small number of microorganism species, plants, and animals such as human. In animals, its expression in the brain has been reported. As a function of GabT, GabT is known to transfer an amino group from γ-aminobutyric acid (GABA) as a substrate to an organic acid in a pyridoxal phosphate-dependent manner.

Further, GabT has also been reported to have transamination (deamination) activity for a relatively broad range of substrates other than GABA, and with GabT derived from E. coli, transamination activity for 3-aminopropyl(methyl)phosphinic acid similar in structure to ethanolamine phosphate has been reported. The ethanolamine phosphate degradation reaction of the equation (1) is accompanied by dephosphorylation, and therefore, is different from known reaction modes of GabT. In view of the above-described similarity, however, GabT has been determined to have a high possibility of being an enzyme that catalyzes the reaction of equation (1).

5. Cloning of Pantoea ananatis gabT Gene and E. coli gabT Gene

A band corresponding to the gabT gene was obtained from the genome of Pantoea ananatis by a known PCR-dependent cloning method, and was ligated to pUC18 for amplification. A DNA sequence analysis was contracted to a custom service provider (Hokkaido System Science Co., Ltd.), and the target gabT gene was confirmed to be obtained. The amino acid sequence of the resulting Pantoea ananatis-derived GabT is shown as SEQ ID NO:1 in the accompanying sequence listing.

Similarly, from E. coli (wt) in the genome of which the existence of the gabT gene has been confirmed, the genome was extracted, a band corresponding to the gabT gene was obtained by a known cloning method, and the band was ligated to pUC18 for amplification. As a result of a DNA sequence analysis, the target gabT gene was confirmed to be obtained. The amino acid sequence of the resulting E. coli-derived GabT is shown as SEQ ID NO:2 in the accompanying sequence listing, and the base sequence of the E. coli-derived gabT gene is shown as SEQ ID NO:3.

6. Construction of GabT Expression System and Confirmation of Enzyme Activity Referential Example 5

The Pantoea ananatis-derived gabT gene, an expression product of which will be abbreviated as P-GabT”, was inserted in the expression vector pET23a to transform the E. coli BL21(DE3) strain. The transformed E. coli was cultured in LB medium, and IPTG was added in a presteady state to induce the expression of P-GabT. Five hours after the induction, centrifugation was conducted to harvest cells. The thus-obtained cells were suspended in a lysis buffer with pyridoxal phosphate contained therein (10 mM pyridoxal phosphate, 100 mM potassium chloride, 1 mM ethylenediaminetetraacetic acid, 100 mM potassium chloride, 1 mM dithiothreitol, 50 mM tris-hydrochloric acid, pH 7.5), and then, cell disruption was conducted by sonication.

The resulting cell lysate was centrifuged to collect a supernatant fraction. The cell lysate before the centrifugation, which will be abbreviated as “total”, and the supernatant fraction, which will be abbreviated as “sup”, were separately added to portions of a tris-hydrochloric acid buffer (final concentration: 20 mM: pH 7.5) containing ethanolamine phosphate (final concentration: 1 mM) and pyridoxal phosphate (final concentration: 1 mM) such that their final volume ratios reached 30%. After the resulting mixtures were separately subjected to the reaction at 30° C., acetaldehyde dehydrogenase (ALDH) and oxidized nicotinamide dehydrogenase (NAD⁺) were added to the respective samples after the reaction according to the manual of the commercial acetaldehyde quantification kit (“F-kit Acetaldehyde”, product of Roche Diagnostics K.K.). Subsequent to the reaction of equation (2), the absorbances at 340 nm were measured. From each absorbance, the acetaldehyde in the corresponding sample after the react ion of equation (1) was quantified, and the ethanolamine phosphate-degrading enzyme activity in the sample before the reaction of equation (1) was determined on the basis of acetaldehyde level. Results are indicated by “P-” in FIG. 5. In the graph, the ordinate indicates the absorbance at 340 nm, and each error bar indicates a standard deviation obtained from thrice repeated measurements.

Referential Example 6

By similar methods and procedures as in Referential Example 5 except for the use of E. coli-derived gabT gene (an expression product of which will be abbreviated as “E-GabT”) in place of the Pantoea ananatis-derived gabT gene, an experiment was conducted to obtain a cell lysate and supernatant fraction. Their ethanolamine phosphate-degrading enzyme activities were measured. Results are indicated by “E-” in FIG. 5.

Referential Example 7

By similar methods and procedures as in Referential Example 5 except that the transformation was conducted using pET23a alone as a negative control, an experiment was conducted. The ethanolamine phosphate-degrading enzyme activities of a cell lysate and supernatant fraction were measured. Results are indicated by “NC” in FIG. 5.

From the results of Referential Examples 5 to 7, in the case of the cell lysates on a left side in FIG. 5, it was possible to confirm high ethanolamine phosphate-degrading enzyme activity on both P-GabT and E-GabT compared with the negative control. In the case of the supernatant fractions on a right side in FIG. 5, on the other hand, both P-GabT and E-GabT had similar enzyme activity as the negative control. Further, also in SDS-PAGE, P-GabT and E-GabT were both confirmed as dark bands at 45 kDa in the case of the cell lysates, but were of similar levels as the control in the case of the supernatant fractions (not shown). From the above results, P-GabT and E-GabT are both considered to exist as inclusion bodies in the precipitate fractions.

7. Determination of Sequences of GabT Domains Referential Example 8

The amino acid sequences of GabTs (aminobutyric acid aminotransferase in the case of mammals) derived from Sulfolobus (Sulfolobus tokodaii str. 7; SEQ ID NO: 4) as an archaebacteria, human (Homo sapiens; SEQ ID NO: 5) and mouse (Mus musculus; SEQ ID NO: 6) as the mammals, and Arabidopsis (Arabidopsis thaliana; SEQ ID NO: 7) as a plant, in addition to Pantoea ananatis and E. coli used in Referential Examples 5 to 7, were aligned using a commercial program (“CLC Sequence Viewer”, prepared by CLC Bio Japan, Inc.). Results are shown in FIG. 6. In FIG. 6, the region boxed by a thick line and containing the 301^(st) to 317^(th) amino acids was extracted as a region of high conservation. It is to be noted that the amino acids shown in rows labeled “consensus” are residues conserved 60% or more.

Referential Example 9

In addition to the GabT sequences of the above-described six organism species, Salmonella (Salmonella enterica subsp. enterica serovar Typhi strain CT18), Clostridium (Clostridium acetobutylicum ATCC 824), Pseudomonas (Pseudomonas putida KT2440), Rhodococcus (Rhodococcus equi 103S), Acinetobacter (Acinetobacter baumanii ACICU), Mycobacter (Mycobacterium avium subsp. paratuberculosis K-10), cattle (Bos taurus), larvacea (Oikopleura dioica), maize (Zea mays) and red pepper (Capsicum annuum) were also verified for the conservation of the domain regions specified in Referential Example 8. Concerning the amino acid sequence ranging from the 301^(st) to the 317^(th), at least 90% of homology was conserved in the above-described 16 organism species. This region was, therefore, defined as a GabT domain.

The GabT domain is an amino acid sequence region consisting of the following amino acid sequence:

GXXXBADEBQXGFZRXG wherein X means an amino acid, B denotes a branched amino acid selected from the group consisting of I, L and V, and Z signifies G or A. The sequences of the GabT domains of the respective organism species are shown in Table 1. In the sequences of Table 1, those different from the corresponding amino acids in the GabT domain defined as described above are indicated by underlining them.

TABLE 1 Homology Organism name/organism species Sequence (%) SEQ ID NO: Pantoea / Pantoea ananatis LMG 20103 GIMLICDEIQSGFGRTG 91 8 E. coli / Escherichia coli str. K-12 substr. GIMLIADEVQSGAGRTG 91 9 MG1655 Sulfolobus / Sulfolobus tokodaii str. 7 GILLVDDEVQMGLGRTG 91 10 Salmonella / Salmonella enterica subsp. GIMLIADEVQSGAGRTG 91 11 enterica serovar Typhi str. CT18 Clostridium / Clostridium acetobutylicum ATCC DIVFIIDEVQAGFGRTG 91 12 824 Pseudomonas / Pseudomonas putida KT2440 GILLIADEVQTGAGRTG 91 13 Rhodococcus / Rhodococcus equi 103S GIVFVADEVQTGFARTG 100 14 Acinetobacter / Acinetobacterbaumannii ACICU GILLVADEVQSGFARTG 100 15 Mycobacter / Mycobacterium avium subsp. DVLFIADEVQTGFARSG 91 16 paratuberculosis K-10 Human / Homo sapiens GGVFIADEVQVGFGRVG 100 17 Bovine / Bos taurus GGVFIADEVQVGFGRVG 100 18 Mouse / Mus musculus GGLFVADEIQVGFGRIG 100 19 Ascidian / Oikopleura dioica GVLTIADEVQVGFGRVG 100 20 Thale cress / Arabidopsis thaliana GGVCIADEVQTGFGRTG 100 21 Corn / Zea mays GGLCIADEVQAGFARVG 100 22 Cayenne / Capsicum annuum GGVCIADEVQTGFGRTG 100 23 

1-18. (canceled)
 19. An ethanolamine phosphate-measuring reagent comprising an enzyme that catalyzes a reaction that forms acetaldehyde from ethanolamine phosphate, wherein the enzyme is a protein having an amino acid sequence region having at least 90% of homology with a GabT domain consisting of a following amino acid sequence: GXXXBADEBQXGFZRXG wherein X means an amino acid, B denotes a branched amino acid selected from the group consisting of I, L, and V, and Z signifies G or A.
 20. The ethanolamine phosphate-measuring reagent according to claim 19, wherein the enzyme is: (1) a protein consisting of an amino acid sequence represented by SEQ ID NO:1 or 2, (2) a protein consisting of an amino acid sequence that in the amino acid sequence represented by SEQ ID NO:1 or 2, one or a few amino acids have been deleted, substituted or added, or (3) a protein consisting of an amino acid sequence having at least 90% of homology with the amino acid sequence represented by SEQ ID NO:1 or
 2. 21. The ethanolamine phosphate-measuring reagent according to claim 19, further comprising a coenzyme for the enzyme.
 22. The ethanolamine phosphate-measuring reagent according to claim 21, wherein the coenzyme is pyridoxal phosphate.
 23. A depression diagnostic agent comprising the ethanolamine phosphate-measuring reagent according to claim
 19. 24. An ethanolamine phosphate-measuring kit comprising the enzyme, which catalyzes the reaction that forms acetaldehyde from ethanolamine phosphate, according to claim 19; and a first container, wherein the kit contains the enzyme in the first container separately.
 25. The ethanolamine phosphate-measuring kit according to claim 24, further comprising a second container with a coenzyme for the enzyme, wherein said coenzyme is contained in the second container separately in the kit from the enzyme in the first container. 